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David Peterzell; The phantom pulse effect: Rapid left-right mirror reversals evoke unusual sensations of phantoms, movements, and paresthesias in the limbs and faces of normals and amputees. Journal of Vision 2008;8(6):830. doi: 10.1167/8.6.830.
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Ramachandran, Altschuler and others have shown that a simple mirror reflection of a moving intact limb causes phantom sensations in normal observers, and reduces phantom limb pain in some amputees. However, mirror therapy is ineffective for many amputees, and neural mechanisms underlying phantom sensations are not fully understood. While attempting to develop more powerful mirror treatments for phantom limb pain, I have discovered that stroboscopic motion and mirror reversals greatly amplify mirror effects in upper-limb amputees and some normal individuals. The “phantom pulse” has been generated in two ways. The first involves using a real-time video image of the observer that flickers between a normal image and a mirror-reversed image at rates varying from 0.5 to 2 cycles/sec (with an 0.2-sec delay). The second involves using Ramachandran's simple mirror with a strobe light, in a dark room. For both methods, movement of one limb causes powerful phantom sensations and a sense of movement in the opposite limb, leading to permanent pain reduction in some amputees. Approximately 50% of normal observers experience mild to moderate phantom-like phenomena using these methods. The reversal or flicker rate that optimizes effects occurs at approximately 1 cycles/sec for most individuals. Normal individuals typically experience some combination of the following in their fingers and hands: tingling, numbness, tickling, pressure, heat, cold, or involuntary movement. In some, these sensations move gradually from the wrist to the shoulder. In perhaps 1 or 2% of individuals, the face tingles. In these most powerful instances, the individual is unable to close just one eye when instructed to “wink.” Use of a temporally-modulated stimulus may enable researchers to (1) conduct psychophysical investigations of mirror-phantom phenomena, and (2) examine physiological correlates these effects using EEG and FMRI. We speculate that neurons with similar transient temporal properties contribute to these profound effects.
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